About 4,000 Cambodian garment workers have protested in Phnom Penh after a factory supplying global brands including GAP and H&M fired hundreds of their colleagues for striking over insufficient wages.

The demonstrators said on Thursday that the Singapore-owned SL Garment Processing factory fired 720 workers a day earlier while more than 5,000 others were suspended after a two-week long strike.

They had walked out claiming intimidation over regular factory inspections carried out by an official flanked by military police.

The workers marched about 10km from the factory to the capital's City Hall to urge the government to intervene in the dispute.

“We want the factory to allow the workers to go back to work,” union leader Ath Thorn, president of the Coalition of Cambodian Apparel Workers Democratic Union, told reporters.

“But if they want to close they factory they must pay compensation to workers according to the law,” he said.

A Cambodian investor in the factory confirmed the staff dismissals to the AFP news agency.

“Their strike was illegal,” Meas Sotha said, adding that the factory had not intended to intimidate workers with the presence of armed police on site.

The company would pay compensation to the sacked workers while the others would be allowed to return to work on Monday, he added.

Disputes frequent

Disputes over wages, safety and conditions in Cambodia's lucrative garment industry are frequent.

The multibillion-dollar industry employs about 650,000 people and is a key source of foreign income for the impoverished country.

Currently workers can earn about $110 a month with overtime.

Concerns over worker safety intensified in May after a ceiling collapse killed two workers at a Taiwanese-owned factory producing shoes for Japanese sports brand Asics.

Cambodian Prime Minister Hun Sen has said protests may imperil the country's garment industry by persuading firms to relocate to Myanmar, Laos and India where labour is cheaper.